Wafer bonding has been developed as a method for bonding large area oxidized or non-oxidized silicon wafers as disclosed by U. Goesele and R. Stengl in U.S. Pat. No. 4,883,215, Entitled "Method for Bubble-Free Bonding of Silicon Wafers". Since wafer bonding makes it possible to bury oxide and implantation layers within the bulk of a monocrystalline silicon wafer and combine monocrystalline silicon wafers, it may be a low cost and highly flexible method for silicon-on-insulator (SOI) and epitaxial applications. Silicon direct bonding (SDB) may also lead to new device structures in the fields of power devices and sensors.
However, in spite of its apparent simplicity there is at least one major obstacle to overcome before wafer bonding can be considered as a reliable technique to produce SOI wafers for electronic devices or for other possible applications. The obstacle is bonding voids or "bubbles" which form at the interface of the mated wafers and are detrimental to the efficacy of the bond. Goesele and Stengl developed a novel and simple procedure relating to this obstacle which serves to eliminate interface bubbles at the mated wafer interface and which does not mandate that the procedure be performed in a Cleanroom.
Interface bubbles in bonded wafers formed by the direct bonding method disclosed by Goesele and Stengl are thought to be caused by dust or other particles and insufficient wafer flatness. While the latter case can be excluded by appropriate wafer specifications, it is difficult to realize totally particle-free wafer surfaces prior to the bonding procedure. It has been found that, even for wafers mated in a Class 1 Cleanroom, unless extreme care has been taken in terms of keeping the time between cleaning and bonding short, almost all of the wafers contain one or more bubbles due to enclosed particles of less than 1 micrometer (.mu.m) in size. Although the complete absence of bubbles may not be necessary for manufacturing power devices from bonded wafers, it is necessary for SOI applications and desirable for all applications. In order to produce completely void-free wafer pairs more complicated techniques rely on high pressure and annealing techniques after wafer bonding and, contrary to findings of some researchers in this field, applicant has not found that the bubbles vanish when wafer pairs are annealed above 1000.degree. C. Centigrade (C). Instead, it was found that basically all the voids introduced during the bonding process at room temperature remain during the annealing step. The procedure developed by Goesele and Stengl as well as other methods based on the careful use of Cleanrooms allow nowadays the preparation of bonded silicon wafer pairs with no bubbles present at the bonding interface directly after room temperature bonding. It has now been discovered, however, that in such initially bubble-free bonded wafers, bubbles are found to form at the bonding interface after this bonded interface is heated to elevated temperatures, as reported by Mitani, Lehmann, Stengl, Feijoo, Goesele and Massoud in the Japanese Journal of Applied Physics in 1991, volume 30 starting on page 615. These interfacial bubbles are thought to be caused by thermally unstable hydrocarbons (denoted here by CH.sub.x) which desorb from the silicon surfaces during a heat treatment and develop a sufficiently high gas pressure to generate bubbles at the interface of the bonded silicon wafers.